This invention relates to a raster output scanner with a multi-beam laser diode and, more particularly, to a raster output scanner which utilizes a zoom system to control the laser diode beam separation.
Referring to FIG. 1, a sagittal view of a conventional multi-beam raster output scanner 10 is shown which utilizes a multi channel laser diode 12, a collimating lens 14, a spot size control aperture 16, pre-polygon optics 18 and a scanning polygon mirror 20, a post polygon optics 22 and a photoreceptor plane 24. The collimating lens 14 receives several beams 26 from different channels of the laser diodes 12. The collimated light beams emerging from the collimating lens 14 start converging towards each other and they all cross each other at the center of the aperture 16.
The spot size control aperture 16 is used for clipping the light. After passing through the spot size control aperture 16, the collimated beams 26, which have crossed each other at the center of the aperture 28, start diverging from each other. The pre-polygon optics 18 focuses each beam 26 individually in the sagittal or cross-scan plane to a spot at a point near a facet of a rotating polygon mirror 20 while in the fast-scan plane the light beam remains collimated when the beam strikes the facet of the rotating polygon mirror.
The rotating polygon mirror 20 causes the reflected light beams to revolve about an axis near the reflection point of the rotating polygon mirror. For the purpose of simplicity, the rotating polygon mirror is shown as line 20 and the reflected light beam from the rotating polygon mirror 20 is unfolded. The post polygon optics images the reflected light beams onto the photoreceptor plane 24. The reflected light beams can be utilized to scan a document at the input end of an imaging system as a raster input scanner or can be used to impinge upon a photosensitive medium, such as a xerographic drum (photoreceptor), in the output mode as a raster output scanner. At the photoreceptor plane 24, there is a distance 30 between the centers of adjacent spots which hereinafter is referred to as "spot separation".
In multi-beam scanning systems, the spot separation 30 is an important element. Simultaneous scanning of multiple spots requires a selection of a precise spot separation at the photoreceptor plane. However, the spot separation is sensitive to fabrication errors and the manufacturing tolerances of the multi-beam light source spacing, pre-polygon optics, post-polygon optics, optomechanical housing, etc. The fabrication errors or the manufacturing tolerances of each individual element or the combination of the elements will cause an error in the spot separation.
Typically, in conventional scanning systems, any adjustment made to compensate for the spot separation errors is not satisfactory since while the adjustment corrects the spot separation, it also causes the spots to go out of focus.
An other approach to correct the spot separation errors is to tighten the tolerances of the various components that contribute to the spot separation errors. However, this recourse is cost prohibitive and impractical.
It is an object of this invention to provide optical means to correct the spot separation errors, caused by the manufacturing tolerances of the optical elements of the raster scanner, in the cross-scan plane.